1. Field of the Invention
The present invention relates to a method of growing a monocrystalline rod, and in particular to a method of growing a high-purity silicon monocrystalline rod having a uniform electric resistivity in the cross-sectional plane perpendicular to the rod axis by an FZ process (floating zone process or floating zone melt process) using a high-frequency induction heating means for heating and melting.
2. Prior Art
Conventionally, as method of growing a silicon monocrystalline rod, the above FZ process and the CZ process (Czochralski process or pulling process) are known.
As is well known, growth of a silicon monocrystalline by the FZ process is carried out as follows:
As shown in FIG. 8 which will be described later, in a chamber 5 a polycrystalline raw material silicon rod 1 having a prescribed diameter is held at the lower end of an upper shaft 10 suspended from above, a prescribed seed crystal 7 is held at the upper end of a lower shaft 8 located below, a high-frequency electric current is passed through a high-frequency heating coil 2 to heat and melt the raw material rod 1 and the seed crystal 7 to fuse them at a constricted part 6, and they are lowered at an extremely slow speed while they are rotated at a prescribed speed so that a melted zone part 4 is moved gradually upward thereby growing a silicon monocrystalline rod 3 having a prescribed diameter.
On the other hand, in the CZ process of growing a silicon monocrystalline, as is well known, a seed crystal having a desired crystal orientation is mounted to the lower end of a pull shaft and the pull shaft is rotated at a prescribed rotational speed and at the same time is pulled up gradually with an end of the seed crystal being dipped in a relatively large amount of a melt of silicon so that a silicon monocrystalline rod having a prescribed diameter is grown and formed.
The distribution of the electric resistivity of the silicon monocrystalline rod grown in the above manner is divided generally into the distribution in the direction of the axis of the growth and the distribution in the cross-sectional plane perpendicular to the axis. The distribution of electric resistivity in the direction of the axis of the growth and the distribution of electric resistivity in the cross-sectional plane in the FZ process and the CZ process will now be discussed below.
First, in the CZ process, when the silicon melt of a liquid phase adhered to the seed crystal solidifies to a silicon monocrystal of a solid phase, segregation of the dopant substance occurs, the concentration of the dopant gradually increases, and the electric resistivity decreases with the growth of the crystal, resulting in an increase in nonuniformity of electric resistivity in the direction of the axis of the growth.
In the FZ process, since a silicon melt is supplied continuously from above to a relatively small limited amount of the silicon melt at the part where the growth takes place, the dopant concentration in the direction of the axis of the growth decreases in a macroscopic sense more greatly than in the case of the CZ process and the distribution of the electric resistivity in the direction of the axis of the growth is made uniform, but on the other hand, in the FZ process, since the amount of the silicon melt at the part where the growth takes place is relatively small and a silicon melt is continuously supplied from above, the dopant substance is taken in irregularly in a microscopic amount with a change in convection in the melt at the growth part, thereby increasing nonuniformity in the distribution of electric resistivity in the cross-sectional plane perpendicular to the axis.
FIG. 2(a) is a graph of an example of the rate A of change of the electric resistivity of a silicon wafer in the cross-sectional plane, which silicon wafer was obtained by growing a silicon monocrystalline rod which had a diameter of 100 mm and whose growth orientation was &lt;111&gt; by rotating it at a speed of 6 rpm and slicing the rod into a wafer having a thickness of 300 .mu.m.
In FIG. 2(a), the rate A of change of the electric resistivity R is defined as follows: EQU A=[(Rmax-Rmin)/Rave].times.100 (%)
wherein Rmax denotes the maximum value of the measured electric resistivity R, Rmin denotes the minimum value of the measured electric resistivity R, and Rave denotes the average value of the electric resistivity R in the wafer plane, and the rate a of variation of the electric resistivity in the cross-sectional plane is defined as follows: EQU a=[(Rmax-Rmin)/Rmin].times.100 (%).
In this case, the reason why the value of the rate A of change of the electric resistivity R is used instead of simply plotting the electric resistivity R is to avoid that the rate A of change of the actual electric resistivity R appears great as the electric resistivity R increases and that depending on the rate a of variation of the electric resistivity R in the cross-sectional plane, the variation of the electric resistivity R can be expressed as one numerical value and the distributions of the electric resistivities R can be assessed by comparing them mutually.
Thus, as apparent from the graph shown in FIG. 2(a), it can be understood that the rate A of change of the electric resistivity R decreases near the central part of the wafer and that its distribution of the electric resistivity in the cross-sectional plane is nonuniform. The value of the rate a of variation of the electric resistivity R in the cross-sectional plane is 22.1%.
Thus, in the growth of an individual silicon monocrystalline rod, that is, in the production of a silicon wafer, it is demanded that the value of the rate a of variation of the electric resistivity R in the cross-sectional plane is small as far as possible and, in some cases of devices which are subject to severe restrictions, the rate a of variation of the electric resistivity R is required to be 3% or below.
Therefore in such a case it is known to take such measures that a prescribed silicon monocrystalline rod is grown by an FZ process without using a dopant substance in the silicon melt and then, for example, the obtained silicon monocrystalline rod is loaded in a nuclear reactor and is irradiated with thermal neutrons so that the doping with a dopant formed by changing .sup.30 Si to .sup.31 P by a nuclear reaction may be effected. This neutron irradiation doping method requires, however, nuclear reaction facilities and therefore is attended with a disadvantage that the production cost of silicon wafers increases considerably. Thus, means of growing industrially a silicon monocrystalline rod wherein the value of the rate a of variation of the electric resistivity R in the cross-sectional plane is small is desired to be suggested.
With respect to the amounts of silicon melts at parts where growth takes place in the FZ process and the CZ process, in the former process the amount is about 1/100 to 1/1,000 of that of the latter process and since it is assumed that the state of convection in the silicon melt is difficult to control artificially not as in the CZ process, it is considered that unevenness of the concentration distribution of the dopant in the cross-sectional plane in the silicon monocrystalline rod formed by the FZ process, that is, the nonuniform distribution of the electric resistivity cannot be obviated.
The convection in the silicon melt included in the growth of a silicon monocrystalline rod by the FZ process involves forced convection caused by the rotation of a seed crystal, natural convection caused by heating by a high-frequency heating coil, and surface tension convection induced by the melt free surface which would increase far more largely in comparison to an increase in the volume of the silicon melt.
To suppress the rates of the natural convection and the surface tension convection as far as possible, it is considered to cause the forced convection to work against these convections, but since, in the FZ process, the amount of the silicon melt at a part where growth takes place is small, the forced convection lacks in strength and therefore secures hardly a desired counteraction to the natural convection and the surface tension convection. Further, although it can be conceived that the silicon monocrystalline rod in the process of the growth is rotated at a higher speed to cause the forced convection to work more strongly, since initially in the process of the growth the weight of the silicon monocrystalline rod itself is supported by the constricted part formed at the lower end of the silicon monocrystalline rod, the constricted part cannot withstand such high-speed rotation at all and there is even a danger that the silicon monocrystalline rod itself in the process of the growth will collapse, which makes this idea impractical.
To obviate these problems, for the growth of a silicon monocrystalline rod by an FZ process, a means of applying a magnetic field in parallel with the direction of the growth to a silicon melt was suggested by N. De Leon, J. Guldberg, and J. Salling: J. Cryst. Growth 55 (1981) 406 to 408 and they reported that when a silicon monocrystalline rod having a diameter of 42 mm was grown and formed by applying a magnetic field of 180 gauss or less approximately in parallel with the axis of the growth, in the silicon wafer obtained therefrom, the rate of variation of the electric resistivity in the cross-sectional plane could be restricted to a lower value.
However, with respect to industrial wafers obtained from a silicon monocrystalline rod grown and formed by an FZ process, wafers having a diameter of 75 mm or over are currently demanded mainly. Thus, the production of silicon wafers having a diameter of 50 mm or less reported by N. De Leon et al. cannot meet the current demand at all.
That is, in the above method proposed by N. De Leon et al., during the growth of a silicon monocrystalline rod having a diameter of about 70 mm or over by an FZ process, when a magnetic field of 180 gauss is applied approximately in parallel with the direction of the growth to the silicon melt, the change in the electric resistivity in the cross-sectional plane of the silicon wafer obtained from the thus grown and formed silicon monocrystalline rod becomes such that the electric resistivity R near the central part of the wafer lowers remarkably, so that the value of the rate a of variation of the electric resistivity R in the cross-sectional plane will exceed 20%.